1. Field of the Invention
The present invention relates to a pre-emphasis adjustment method and more particularly, to a pre-emphasis adjustment method for controlling the pre-emphasis intensity provided at the transmission side of a signal transmission line based on a received signal. The present invention also relates to a pre-emphasis adjusting system having a pre-emphasis intensity adjusting function.
2. Description of the Related Art
Generally, in a transmission line, transmission signals in a high frequency range involve a larger attenuation rate. Accordingly, when signals of a plurality of frequency components are transmitted from the transmission side at the same level for all of the frequency components in the signal, there occurs a difference in the amplitude of the signal between the higher frequency components and the lower frequency components at the receiving side which receives the signals through the transmission line. Normally, to reduce the difference in the signal level or amplitude between the different frequencies at the transmission side, a pre-emphasis technique is used in which the signal level of the lower frequency components is lowered beforehand, or the signal level of the higher frequency components is raised beforehand and then a signal including the lower signal components and higher signal components is transmitted (for example, refer to JP-A-06-177798). In this manner, at the transmission side, the signal level is differentiated between the lower frequency components and the higher frequency components. Thereby, the signal level at the receiving side can be maintained substantially constant among all the frequency components in the signal irrespective of the frequencies.
Regarding the pre-emphasis, it is important to create the pre-emphasis of appropriate intensity according to the characteristics, etc., of the transmission line. FIG. 3A shows the configuration of a conventional automatic adjustment system for adjusting the pre-emphasis. An adjustment control unit 54 of the receiving side transmits a pre-emphasis intensity control signal to a transmitting circuit 51, and an automatic control unit 55 of the transmitting circuit 51 controls the pre-emphasis intensity of the transmission signal to be transmitted from the transmitting circuit 51.
A reception judging unit 53 judges whether or not the signal transmitted by the transmitting circuit 51 can be received correctly at the receiving circuit 52. Then, the reception judging unit 53 obtains a receivable time width, or time interval, in which the transmission signal can be received, while controlling the delay of the timing of a clock signal for receiving the signal, as shown in FIG. 3B.
In the automatic pre-emphasis adjustment system shown in FIG. 3A, the above procedure is tried for all of the adjustment values of the pre-emphasis intensity, and the pre-emphasis intensity of the transmitting circuit 51 is adjusted so that the receivable time width assumes a maximum. This is exemplified in FIG. 3C, wherein the pre-emphasis intensity is lowered, for example, from the maximum level toward a minimum level of the pre-emphasis intensity in a step-by-step basis, until an optimum pre-emphasis intensity is detected.
If the signal transmission bit rate in the above conventional pre-emphasis technique is on the order of 10-Gbps class, the number of adjustment steps of the pre-emphasis intensity is extremely large in order to correctly receive the transmission signal by the receiving circuit 52. For example, the receiving circuit 52 requires 100 steps or more for the procedure for determining the receivable time width. Therefore, if the signal transmission bit rate is high, and if the pre-emphasis intensity that maximizes the receivable time width is to be obtained by trying all of the adjustment values, it takes an extremely large time length to complete the pre-emphasis adjustment.